Surface-emitting laser diodes or Vertical-Cavity Surface-Emitting Lasers (VCSEL) are semiconductor lasers, in which light emission occurs perpendicular to the surface of the semiconductor chip. Compared with conventional edge-emitting laser diodes the surface-emitting laser diodes have numerous advantages such as low electrical power consumption, the possibility of direct monitoring of the laser diode on the wafer, ease of coupling to a glass fiber, production of longitudinal single mode spectra and the possibility of connecting the surface-emitting laser diodes together to form a two-dimensional matrix.
In the field of fiberoptic communications technology—because of the wavelength-dependent dispersion or absorption—devices producing radiation in a wavelength range from about 1.3 to 2 μm, and in particular wavelengths of about 1.31 μm or 1.55 μm, are needed. Longwave laser diodes with useful properties, particularly for the wavelength range above 1.3 μm, have been produced from InP-based connecting semiconductors. GaAs-based VCSELs are suitable for the shorter wavelength range of less than 1.3 μm.
A continuous wave VCSEL which emits at an output of 1 mW at 1.55 μm has been constructed of an InP substrate with metamorphic layers or mirrors (IEEE Photonics Technology Letters, 11(6), June 1999, pp. 629-631). Lateral wave guiding is achieved by proton implanting.
A VCSEL produced in a single epitactic growth process with an output of 0.45 mW at a wavelength of 1.5 to 1.6 μm with a metamorphic mirror on the light emitting side is known from “High Performance 1.6 μm Single-Epitaxy Top-Emitting VCSEL”, (Conference on Lasers & Electro-Optics (CLEO) 2000, San Francisco, USA, Post-Deadline Paper CPD 12, pp. 23-24). Current and wave guiding were achieved by selective oxidation.
A VCSEL with an air-semiconductor mirror (InP—air gap Distributed Bragg Reflectors (DBRs)) was proposed in IEEE ISLC 2002, pp. 145-146. A tunnel junction was applied between the active zone and the upper DBR mirror, whereby a current was achieved by under-etching the tunnel junction layer. The air gap surrounding the remaining tunnel junction area serves as a waveguide for the optical field.
A VCSEL with antimonide-based mirrors, in which an under-etched InGaAs active zone is enclosed by two n-doped InP layers, adjoined by AlGaAsSb-DBR mirrors, is known from the 26th European Conference on Optical Communication, ECOC 2000, “88° C., Continuous-Wave Operation of 1.55 μm Vertical-Cavity Surface-Emitting Lasers”. The selective under-etching in this case brings about lateral wave guiding.
The optimum properties in terms of performance, operating temperature range, single mode power and modulation bandwidth, however, are found in VCSELs with buried tunnel junctions (BTJs). The manufacture and structure of the buried tunnel junction will be described hereinafter with reference to FIG. 1. A highly doped p+/n+ pair of layers 101, 102 with a small band spacing is produced by molecular beam epitaxy (MBE). The actual tunnel junction 103 is formed between these layers. By reactive ion etching (RIE) a circular or elliptical area is shaped, which is essentially formed by the n+-doped layer 102, the tunnel junction 103 and some or all of the p+-doped layer 101. This area is overgrown with n-doped InP (layer 104) in a second epitaxy procedure, so that the tunnel junction 103 is “buried”. The contact area between the overgrown layer 104 and the p+-doped layer 101 acts as a barrier layer when voltage is applied. The current flows through the tunnel junction with resistances of typically 3×10−6 Ω cm2. In this way, the current flow can be restricted to the actual area of the active zone 108. Moreover, little heat is generated, as the current flows from a high-resistance p-doped layer to a low resistance n-doped layer.
The overgrowing of the tunnel junction leads to slight variations in thickness which have an unfavorable effect on lateral wave guiding, with the result that the production of higher lateral modes is made easier, particularly in the case of larger apertures. Therefore, only small apertures can be used with less corresponding laser power for single mode operation, which is required in fiber optic communications technology.
Examples and applications of VCSELs with buried tunnel junctions can be found, for example, in “Low-threshold index-guided 1.5 μm long wavelength vertical-cavity surface-emitting laser with high efficiency”, Applied Physics Lett., 76(16), Apr. 17, 2000, pp. 2179-2181; in “Long Wavelength Buried-Tunnel-Junction Vertical-Cavity Surface-Emitting Lasers”, Adv. in Solid State Phys. 41, 75-85, 2001; in “Vertical-cavity surface-emitting laser diodes at 1.55 μm with large output power and high operation temperature”, Electronics Letters, 37(21), Oct. 11, 2001, pp. 1295-1296; in “90° C. Continuous-Wave Operation of 1.83 μm Vertical-Cavity Surface-Emitting Lasers”, IEEE Photonics Technology Letters, 12(11), November 2000, pp. 1435-1437; and in “High-speed modulation up to 10 Gbit/s with 1.55 μm wavelength InGaAlAs VCSELs”, Electronics Letters, 38(20), Sep. 26, 2002. Lateral wave guiding is provided in these examples by lateral variation in the resonator length.
The structure of the InP-based VCSEL discussed in the above-mentioned literature will be briefly explained below with reference to FIG. 2, starting from the structure of the buried tunnel junction in FIG. 1 described above.
The Buried Tunnel Junction (BTJ) in this structure is arranged in reverse relative to FIG. 1. The active zone 106 is situated above the tunnel junction having a diameter DBTJ determined by the p+-doped layer 101 and the n+-doped layer 102. The laser radiation travels in the direction indicated by arrow 116. The active zone 106 is surrounded by a p-doped layer 105 (InAlAs) and an n-doped layer 108 (InAlAs). A front mirror 109 above the active zone 106 consists of an epitaxial DBR with 35 pairs of InGaAlAs/InAlAs layers, producing a reflectivity of about 99.4%. A rear mirror 112 consists of a stack of dielectric layers as DBRs and ends with a layer of gold, producing a reflectivity of nearly 99.75%. An insulating layer 113 prevents direct contact of the n-InP layer 104 with the p-end contact layer 114, which is generally comprised of gold or silver (in this context see DE 101 07 349 A1). Reference numeral 111 designates the p-end contact layer which is annular in structure.
The combination of the dielectric mirror 112, the integrated contact layer 114 and heat sink 115 results in a significantly increased thermal conductivity compared to epitaxial multi-layer structures. Current is injected through the contact layer 114 or through the integrated heat sink 115 and the n-end contact points 110. For further details of the manufacture and properties of the VCSEL types shown in FIG. 2, express reference is made to the literature mentioned above.
In the proposed VCSEL diodes, particularly for the wavelength range between about 1.3 and 2 μm, there is a need to be able to adjust the lateral radiation profile thereof within wide ranges by lateral wave guiding. Here, too, manufacturing should take place with the usual epitaxial overgrowth, and for this reason Al-free InP-based VCSELs, in particular, are suitable for wavelengths above 1 μm.
In GaAs-based VCSELs, which can only be used in wavelength ranges below about 1.3 μm, lateral wave guiding is produced by selectively oxidized AlAs layers (in this context see “Advances in Selective Wet Oxidation of AlGaAs Alloys”, in IEEE Journal of Selected Topics in Quantum Electronics, 3(3), June 1997, pp. 916-926). The VCSEL discussed therein consists of multiple layers of GaAs—AlGaAs, produced epitaxially by Metal Organic Vapour Phase Epitaxy (MOVPE). By wet oxidation of the AlGaAs layers, buried oxide layers are formed which leave an unoxidized aperture open in the center of the VCSEL. This method has not hitherto been successfully applied to the InP-based VCSELs, as AlAs cannot be applied or can only be applied in layers that are too thin because of mismatching of the lattice constants, and other oxidizable materials such as, for example, AlGaSb have hitherto not produced an oxide layer of sufficient quality. Therefore, with long wave VCSELs, other methods of lateral wave guiding have been used, such as for example lateral variation of the resonator length, selectively etched layers, proton implanting or metamorphic AlAs layers, as explained above in connection with references cited.